The possibility that terrorists might detonate nuclear fission bombs has seized both the general public and policy communities’ attention since the September 11, 2001 terrorist attacks. Experts differ significantly on how likely or even plausible the threat might be, with well-informed, thoughtful analysts adopting views across the spectrum. Even among these diverging views, there is a relative consensus that terrorists are not likely to develop their own fissile material for nuclear weapons, and also that building even a crude nuclear device would require considerable expertise. Those working inside military, government, or commercial establishments may provide crucial materials or skills to terrorist groups who would otherwise be hampered in realizing nuclear attack motivations. More generally, insiders have the potential to pose significant nuclear threats. For example, causing a consequential radiation release from a nuclear reactor via external attack is challenging, but far less so with an insider accomplice.